Landscape Walls and Rubble Trench Foundations

It must be something in the air. I have received a bunch of emails about building landscape walls in the last week. This after a long drought of such questions. So, I guess it is time to talk about them again. The majority of the questions I have been getting are around the foundation system and the restrictions of building a landscape wall with bales.

(Note that in the diagram below, the foundation is not properly insulated. I use this photo solely as an example of a deep rubble trench assembly)

Let’s start with the foundation. I prefer a rubble trench foundation for landscape walls as the need for concrete is low and the need for positive drainage is high. I want water to move away from the wall. I also don’t want the wall to heave and shift with water and freezing cycles. A rubble trench foundation does a great job of protecting the wall from such concerns and also provides a strong foundation for the wall to sit on. Check out the image of a standard rubble trench above. Note that the gravel goes below the frost line and that the drain in the bottom moves any water away from the wall. It is best to slope the bottom of the trench with a 1/8″ per 1′ slope from end to end. On a long wall, this can be a big difference in depth from the start to the end of the wall, so be prepared for that. The drain should be wrapped in soil fabric and then covered with clean 1 1/2″ river rock. The round river rock is best because it does not need to be compacted mechanically. It will compact on its own under the weight of the material above it. Being that you are surrounding a plastic pipe with the rock, this is a good idea as a mechanical compactor that close to the pipe would probably break it. Once you have the river rock in place, you can add recycled rip rap (old concrete scraps) or rock. I prefer to use 3/4″ minus gravel and then compact it in 4″ lifts as I fill the trench. For me, compaction is key and rip rap is hard to compact well. You can use the ri[p rap and then pour a very wet concrete slurry into the trench to bond all the rip rap together. The slurry flows into the voids between the rip rap and makes one big solid unit of the material. Be sure to get the mix ratio right on the slurry or you will end up with a bunch of cracked, flaky junk in the trench.

The question of how to top that trench is the next issue. A standard application, as shown in the above image, is to use a steel rebar reinforced concrete cap. This cap serves a few purposes. First, it ties the whole system together in one solid piece. Second, it provides a base for the structure above it and gets that structure off of the ground. Third, it provides a place to attach hold downs and anchors for the above structure. This is a simple and accepted method in most jurisdictions. The use of concrete is obviously not the most green application, so there are other options. On a house, I would likely stay with the concrete cap or grade beam, because of the uplift resistance it provides. For a landscape wall, I am willing to let that go and move into a more green concept.
Another option is to use earth bags. These are poly bags, like standard sand bags, filled with dirt and a small amount of concrete to help them set. Although not a great picture, you can see above how they are stacked on top of the rubble trench to provide the lift above grade for the structure and how they create a surface for the wall to be built on. The bags in this picture need to be tamped down in place such that they end up level and fulyt compacted. This is a bit harder than leveling a concrete grade beam, but worth the effort to stay “green.” One thing earthbags don’t provide is any resistance to uplift forces so anchor bolts are not an option with this system. That is fine on a small landscape wall, but not something I would recommend on a house as I mentioned above.

The biggest limitation with landscape walls is the out of plane forces. This is the loads, like wind, that are placed on the surface of the wall that may make it fall over. You must be careful to design for this. As the wall gets taller, you will need to add buttress walls to support the out of plane direction. I have another blog post about buttress walls so I won’t go into that again here. Nonetheless, be aware of the need to add them if your wall starts to get tall. You can limit or avoid buttress walls by curving your landscape wall so there are no straight, unsupported runs. The curves will actually create support for themselves along the length of the wall.

Another limitation is the weather resistance of the wall. As you can see on the cob wall at the start of this post, roofs are a good idea to protect the wall from weather. That said, there is nothing wrong with building a wall without a roof as long as you don’t mind the eventual rot of the straw in the wall as long as you plan for it. If you use steel mesh to reinforce the plaster and strong plaster (either lime or cement based) then the eventual rot will not be a concern. The mesh and plaster will ultimately support themselves even after the bales rot out some 40 years from now. Being that the bales are not used for insulation in this application, they can be considered natural plaster forms and their sacrifice is acceptable. If you want to delay that rot, consider using a pond liner over the top of the wall extending down the top course. Place the liner under the plaster mesh so that the plaster is still attached to the steel for the reasons discussed above (strength and longevity).

Have fun with this project. You can do all kinds of things with landscape walls and create really special spaces in your yard or garden.

About Andrew Morrison

Andrew has a passion for straw bale construction that is matched only by his desire to teach his knowledge to others. He has a wealth of experience in designing and building conventional, tiny, and straw bale homes. After years of building, he has moved his practice entirely to consulting and teaching. He shares his knowledge with thousands of people via his DVD series, this website and the roughly six hands-on workshops he teaches each year.

12 Responses to Landscape Walls and Rubble Trench Foundations

Your cross-section illustration of a slab over a rubble trench foundation does not reflect an understanding of how to properly insulate a slab for a residence, and if it is intended to describe a landscape wall, the illustration seems inappropriate. As your illustration shows, the slab under the wall is exposed to the outside. If for a residence, this construction detail will create a “thermal nosebleed”, providing a direct path between the inside conditioned space and the outside cold. This illustration is misleading and irresponsible for someone claiming to be an expert… Andrew, is this really how you build your straw bale foundations, either for a landscape wall or a residence??

Eric,
Thanks for pointing that out. You are correct and I was a bit hasty in using the illustration. This is actually an illustration from Wikipdeia that I used mainly to show the deep trench with the drain at the bottom for the purposes of giving people an idea of what a rubble trench looks like. I see now that I was somewhat inappropriately blind to the rest of the picture. I see your point now that I have stepped back to look at it.

No, this is not how I build my slabs for homes; however, for a landscape wall, there is no need to worry about the concrete grade beam being exposed to the elements. In fact, one of its jobs is to raise the bales off of the ground, thus exposing itself to the elements. I will put a note under the photo that reflects your comments as you are right on with them. Thanks for the heads up and watchful eye.

I need guidance in creating a large straw bale “rock formation” of faux concrete rock on an Arizona hillside (1:5 grade). Existing hillside has a 1-2″ fine gravel cover over solid calache rock. Calache rock looks and behaves like hard white chalk and is unattractively amorphous. My plan is to erect a straw bale wall 6 feet high at the crest of the hill on a filled cinder block foundation with rebar driven into the underlying calache and the holes filled with cement. The rebar would protrude up 12 inches into the first layer of bales. This foundation would be under the wall bales. By cementing the block to the ground calache along the crest of the hill, any water flow would be forced around the wall. Could I then avoid having a footer under the adjacent bales which compose the stair step descending layers of artificial rock? I would plan to cover the whole top of the multi height “rock formation” with 5 mil plastic sheeting before sewing on the chicken wire through the sides of the bales. This is a non-weight bearing structure for aesthetic purposes. As you mention in the article above: “The mesh and plaster will ultimately support themselves even after the bales rot out some 40 years from now.” So really it shouldn’t matter if the straw rots out in even a year or two? If that would cause a stink, however, I wouldn’t like that. Can the straw bales be treated with some kind of antimicrobial agent to stop decay by bacteria or mold? I would appreciate your thoughts.

Hoping for a reply and some guidance. Note that the hillside would be covered with straw bale “rocks” with some pockets between bales to leave space around mesquite trees already growing on the hillside. (Somehow they manage to grow in the caliche rock. Sorry about the previous misspelling.)

Sorry for the long delay Don. I’ve just been very busy as of late. The bales won’t likely rot if you use a pond liner over the top and under your mesh. I suggest you use a welded wire mesh (16 gauge 2″x2″ would be good) to reinforce the wall as it has structural strength that the chicken wire does not. You’ll need to provide a place to nail the mesh at the bottom of the wall on both sides, but other than that, you won’t need to add any additional cornet with the design you have described. Again, as long as you can provide the wood nailer attachment point. At the top of the, add chicken wire OVER the 2×2 mesh and tie wire it to the 2×2. It’s purpose is to provide extra support for the plaster where the pond liner is present.

We’re in the planning stages of using a rubble trench foundation for a house and in our desire to avoid concrete altogether, are considering granite block sills with 8×8 Hemlock bolted to it (which would make the grade beam dis-continuous.) Is this an issue/concern/mistake in anyone’s opinion? Thanks for your thoughts.

I would prefer a continuous foundation if it were mine. You could use mortar to at least join the granite blocks together. That would be a step in the right direction. And after all, my parents’ house is Maine was built in 1704 or something like that and it had a wooden foundation that simply laid on the ground. The house did just fine for all those years!

I like the pond liner better because it is much more durable, especially during construction as building paper tends to rip more easily. I extend it down about halfway over the top bale. If you go too far down the sides, it will make it hard for plaster to properly adhere.